1. Field of the Invention
The present invention relates to a color image forming apparatus such as a color printer, color copying machine, or the like and, more particularly, to a tone correction table generation method and apparatus, which control the tone characteristics that influence the color reproducibility of an image, and an image forming method and apparatus using a tone correction table.
2. Description of the Related Art
In recent years, color image forming apparatuses such as color printers, color copying machines, and the like, which adopt an electrophotography system, ink-jet system, and the like, are required to enhance the image quality of output images. Especially, tonality and its stability impose a large influence on human decision about a good or bad image. Furthermore, upon enhancement of image quality, image quality enhancement in terms of colors such as improvement of the color reproducibility, color matching between images to be printed by different apparatuses, that between an image to be displayed on a display such as an LCD or the like and that to be printed, and so forth is required. To meet such requirements, for the purpose of improving the tonality and its stability, various tone correction table generation methods and calibration methods for obtaining desired tone characteristics by changing the contents of such tone correction tables have been proposed.
For example, Patent reference 1 discloses a method of generating a tone correction table to have tone characteristics in which the brightness level changes linearly. Patent reference 2 discloses a method of generating a tone correction table to have tone characteristics in which an accumulated color difference changes linearly. Patent reference 3 discloses a method of controlling density and tone characteristics by correcting a calibration table in consideration of ternary colors by combining cyan (C), magenta (M), and yellow (Y). Patent reference 4 discloses a method of generating a tone correction table so that an image after correction has tone characteristics in which the brightness level of black (Bk) changes linearly and the chroma or chromatic levels of secondary colors of CMY change linearly.
[Patent reference 1] Japanese Patent Laid-Open No. 2002-292936
[Patent reference 2] Japanese Patent Laid-Open No. 2003-324619
[Patent reference 3] Japanese Patent Laid-Open No. 2003-107835
[Patent reference 4] Japanese Patent Laid-Open No. 2004-205701
However, upon execution of tone control based on a brightness value by the aforementioned conventional tone correction table generation method, a brightness change amount becomes small since the difference between the brightness values of a lightest portion (white portion) and a darkest portion (solid color portion of yellow) is small in, for example, Y (yellow). In such a case, precise tone characteristics cannot often be realized due to the influence of calorimetric errors and inability to perform fine tone separation due to the small brightness difference. On the other hand, upon execution of tone control based on a chromatic value by the conventional tone correction table generation method, an image signal value increases but the chromatic value does not always increase in terms of a chroma change in continuous tone reproduction from a lightest portion (white portion) to a darkest portion (solid color portion) in, for example, C (cyan) and M (magenta). Conversely, even when the image signal value increases, a chromatic value which must increase as the tone level is getting closer to the solid color portion often decreases. Hence, tone control based only on the chromatic value is often disabled depending on the characteristics of color materials. When executing tone control based on a color difference value as the conventional tone correction table generation method, when a tone correction table has characteristics in which the chromatic value decreases as the image signal value increases, the color difference value undesirably decreases at a tone level where the color difference value must increase with increasing image signal. For this reason, the tone control based on the color difference value is often disabled.